Phytoplankton structure is more influenced by nutrient enrichment than by temperature increase: An experimental approach upon the global changes in a shallow subtropical lake

portuguesA mudanca climatica e considerada uma das maiores ameacas aos ecossistemas aquaticos em todo o mundo. Entre as mudancas esperadas, incluindo a dinâmica do fitoplâncton, o crescimento de cianobacterias esta entre os mais citados. O objetivo deste estudo foi avaliar os efeitos isolados e combinados do aumento de temperatura e nutrientes na estrutura da comunidade fitoplanctonica em um grande lago raso subtropical oligo-mesotrofico (Lagoa Mangueira, sul do Brasil). O estudo foi realizado em microcosmos para simular possiveis cenarios de mudanca climaticas, sozinhos e em combinacao com enriquecimento de nutrientes (N e P), a partir de amostras de inverno e verao. Em ambas estacoes, a biomassa total do fitoplâncton, a riqueza e diversidade de especies nao variaram significativamente sob a manipulacao isolada da temperatura. O aumento da temperatura elevou significativamente a biomassa de cianobacterias em experimentos de inverno. No verao, a biomassa total do fitoplâncton, a riqueza de especies, Bacillariophyceae e Cyanobacteria aumentaram significativamente sob o enriquecimento de N e P. A interacao entre aumento de temperatura e adicao de nutrientes nao promoveu efeito significativo sobre os atributos fitoplanctonicos estudados. O aumento isolado da temperatura nao foi forte o suficiente para promover mudancas estruturais na comunidade fitoplanctonica. Entretanto, a adicao de nutrientes (N e P) demonstrou influenciar mais significativamente na maioria dos atributos do fitoplâncton, embora nao tenha sido observado o recrutamento de cianobacterias formadoras de floracoes. Nesse sentido, mais estudos enfatizando a resiliencia dos ecossistemas aquaticos quanto as mudancas climaticas podem contribuir para a conservacao e manejo desses ambientes. EnglishClimate change is considered one of the greatest threats to aquatic ecosystems around the globe. Among the expected changes, including the dynamics of phytoplankton, the growth of cyanobacteria is often cited. This study evaluated the individual and combined effects of increased temperature and nutrients on phytoplankton community structure in a large, oligo-mesotrophic subtropical shallow lake (Mangueira Lake, southern Brazil). The study was conducted in microcosms to simulate possible scenarios of climate change, alone and in combination with nutrient enrichment (N and P); water samples were collected in winter and summer. Phytoplankton total biomass, species richness and diversity did not vary significantly with manipulation of temperature alone, either in winter or in summer. Only cyanobacteria biomass increased significantly with an increase in temperature, in the winter experiments. In summer, phytoplankton total biomass, species richness, Bacillariophyceae and Cyanobacteria increased significantly with enrichment of N and P. The interaction between temperature increase and nutrient addition did not significantly affect phytoplankton attributes. Temperature increase alone was not sufficient to cause structural changes in the phytoplankton community. Instead, nutrient addition (N and P) proved to be the most significant influence on most phytoplankton attributes, even though turnover of bloom-forming cyanobacteria was not observed. Further studies on the resilience of aquatic ecosystems to climate changes may contribute to the conservation and management of these environments